I'm afraid I'm kind of hijacking your post rather than answering it, as I'm also having some problems with an umbrella tree, but maybe we can work find the answers together!
From what I understand though, you have to be a little careful with replanting umbrella trees, I think they should be in quite small pots (about an inch of space between the root ball and the pot), but it depends how cramped yours is. Not sure about the other issues though, but I'm sure someone can help.
My problem is that my once thriving umbrella tree (the variety with the smaller leaves, is it Hawaiian?) has begun to drop leaves. They don't seem to yellow or anything, just fall off, and it's starting to really look sad now. I water when the soil is a little dry, and fertilise regularly. The only thing I can think of is that it isn't getting enough light, but it seemed to survive the Finnish winter last year so I don't know why it wouldn't do so this year as well? I currently have it on the windowsill to try and maximise it's light, but all our windows face the same way, and none get direct light at any time.
Is there anything else I can do to try and save the poor thing?